Atrial fibrillation is the most common arrhythmia with a high risk of stroke and stroke-associated mortality and morbidity. The prevalence of AF and stroke, increases markedly with age and alcohol abuse. The classic pathophysiological concept is atrial mechanical stasis during AF promotes thromboembolism formation. Thus, antithrombotic therapy has been a key component of AF management. However, due to the stroke-bleeding dilemma, and the varying risk of thromboemobolic events, only about half of all AF patients receive antithrombotic therapies. Emerging clinical findings are challenging our long-standing AF-stroke dogma, leaveing the causal link between AF and thrombogenesis even more baffling. The goal of this proposal is to fill this knowledge gap, establish a previously unrecognized crosstalk between heart and platelets through circulating microparticles containing heart-origin activated stress molecule JNK2 and reveal the underlying mechanism of the dual functional role of cardiac JNK2 in both thrombogenesis and AF development. Our intriguing preliminary findings suggest that age- and alcohol-driven JNK activation in the heart is mechanistically linked to the platelet activation and thus increased thrombogenesis. This is potentially a paradigm-shifting concept. Next, we found heart cells shed JNK-microparticles (JNK-MPs) and these MPs could then interact with platelets through an action of JNK2-specific endocytosis. Consequently, increased platelet JNK2 could lead to abnormal platelet Ca homeostasis and increase resting platelet reactivity. All these intriguing preliminary results and our previous findings point to a previously unrecognized JNK2-signaling crosstalk between the heart and platelets. In this proposal, we will use complementary electrophysiological approaches (dual voltage/Ca optical mapping, intravital confocal platelet Ca imaging, in vivo atrial painting gene transfer, ex vivo platelet aggregation, in vivo thrombus formation, and single IP3R channel recording) and biochemical techniques in intact atria, platelets, and even in single channels to gain a comprehensive picture of the relationship between cardiac JNK2, thrombogenesis and AF risk. The JNK2 actions on AF propensity, platelet Ca handling, and resting platelet reactivity will be dissected using several novel cardiac specific inducible Tg mouse models with manipulated JNK2 (JNK1) activity (activated or inactivated) and aged animals with and without atrial-specific JNK2 inhibition using a unique in vivo atrial painting gene transfer technique. To potentially translate the results from animal models to humans, we will perform selective studies in viable human platelets and hearts from organ donors. Our specific aims are: 1) Define the link between cardiac JNK2 (cJNK2) activation, platelet function, thrombogenesis and AF and 2) Delineate how the heart talks to platelets through circulating JNK2-MPs, governing platelet Ca handling and hyper-reactivity. This proposal integrates important functional measurements and fundamental mechanistic studies along with appropriate alternative approaches. Cardiac specific interventions (in vivo atrial gene transfer & genetic JNK inhibition) that limit cardiac JNK2 activity will be tested as proof-in-principle studies and explored as potential therapeutic options to prevent and/or treat thrombogenesis and AF.